1. Field of the Invention
The invention relates to compositions that can be applied to various substrates and cured with ultraviolet (UV) light to form a conformal coating on the substrate, particularly where the substrate is a printed wiring board.
2. The Prior Art
After electronic components have been mounted on printed wiring boards, a conformal coating is usually applied to the boards to protect the components from hostile environments such as high humidity and the presence of corrosive vapors. Conventional solvent-based conformal coating compositions produce coatings having good properties, but the compositions require a long time to dry (at least 5 minutes) at high temperature (usually 150.degree. C. or higher) and harmful solvents are evolved during the curing process. The best conformal coating today is considered to be "Parylene" a trade product of Union Carbide; however, "Parylene" has to be processed by vacuum pyrolysis deposition, a very expensive and patented process.
Conformal coatings that are UV curable have the advantages that they can be cured rapidly (in seconds) with no volatile organic compounds being emitted, so they are well suited for an automated electronic assembly line. However, there is an inadequate cure due to shadowing by the components. U.S. Pat. Nos. 4,424,252 and 4,668,713, for example, disclose a secondary cure mechanism to cure these shadow areas.
In spite of these disclosures, an easily processed solventless, fast curing UV curable conformal coating composition which has the appropriate viscosity, has good shelf life, has low moisture permeability and good electrical properties, and is capable of forming a coating having the required properties including a shadow cure, has not been made.